1. Field of the invention
This invention relates to an effluent-gas-scavenger system especially adapted for use with a process tube wherein multiple wafers of semiconductor material are treated with gases and additives sequentially introduced to the tube under closely-controlled conditions of pressure and of high temperature. Each "charge" of semiconductor material is very valuable and must be processed with precisely-controlled quantities of gases and additives, some of which may be toxic. Accordingly, safe and efficient disposition must be made of the spent gases which exit the process tube after passing the charge of semiconductor material. Moreover, it is important to minimize any "back diffusion" of air or other ambient gas or pollution from the environment of the process tube upstream into the exit portal of the process tube and thence into contact with any of the charge of semiconductor material.
2. Description of the Prior Art
U.S. Pat. No. 5,118,286--Sarin shows an apparatus for preventing exhausted reactant gas, derived from a semiconductor-process tube, from mixing with ambient air in the vicinity of the process tube. That patent discloses the use of an exhaust gas tube for drawing off exhausted reactant gas from an inlet near the downstream end of the process tube and for conveying the exhausted gas through a control valve to a gas scrubber. A closed loop system is provided for controlling the valve to maintain a predetermined small differential between the gas pressure near the downstream end of the process tube and the pressure of the ambient air just outside the end of the process tube.
In the apparatus of Sarin, the heat content of the exhausted reactant gas goes with it through the exhaust gas tube to the scrubber. There is no provision for substantially removing the heat content of the exhausted reactant gas for separate disposition. Nor is it explained how a seal can be maintained between the exhaust tube 30 and inlet 30A, on the one hand, and the process tube 12, on the other. The latter must operate at a very high temperature, and would resist the formation of a tight and permanent seal with any other body.
U.S. Pat. No .4,976,610--Yates discloses the use of a movable quartz "elephant tube" (process tube) 12, to which is connected a return scavenger exhaust tube 34 for feeding back the reactant gas to the jacket around elephant tube 12. No means is provided for removing surplus heat from the exhausted reactant gas and for separately disposing of such surplus heat. It is not apparent how backstreaming exhausted reactant gas is prevented from contaminating the charge of semiconductor material in the elephant tube.